As a technique of introducing impurities into the surface of a solid sample, there is known a plasma doping method that ionizes impurities to introduce into a solid at a low energy (see a patent document 1, for example). FIG. 8 shows the schematic configuration of a plasma processing apparatus used in the plasma doping method as the impurity introducing method of the related art described in the patent document 1. In FIG. 8, a sample electrode 6 for placing a sample 9 configured by a silicon substrate thereon is provided in a vacuum chamber 1. A gas supply device 2 for supplying gas as doping material containing a desired element such as B2H6 within the vacuum chamber 1 is provided and further a turbo-molecular pump 3 for reducing the pressure within the vacuum chamber 1 is provided, whereby the pressure within the vacuum chamber 1 can be kept at a predetermined value. Microwave is irradiated within the vacuum chamber 1 from a microwave guide 41 via a quartz plate 42 serving as a dielectric window. Due to the mutual action between the microwave and a DC (direct-current) magnetic field formed by an electromagnet 43, magnetic field microwave plasma (electron cyclotron resonance plasma) 44 is formed within the vacuum chamber 1. A high-frequency power supply 10 is coupled to the sample electrode 6 via a capacitor 45 so as to control the voltage of the sample electrode 6. The gas supplied from the gas supply device 2 is introduced within the vacuum chamber 1 from a gas introduction port 46 and exhausted to the turbo-molecular pump 3 from an exhaust port 11.
In the plasma processing apparatus thus configured, the doping material gas, for example, B2H6 introduced from the gas introduction port 46 is converted into plasma by a plasma generating means configured by the microwave guide 41 and the electromagnet 43, and boron ions within the plasma 44 are introduced in the surface of the sample 9 by the high-frequency power supply 10.
In the method and apparatus for plasma doping, a method of measuring a high-frequency current to be supplied to the sample electrode is proposed as a method of controlling a doping quantity. FIG. 9 shows the schematic configuration of an example of an apparatus for realizing such a method. In FIG. 9, the sample electrode 6 for placing the sample 9 configured by the silicon substrate thereon is provided in the vacuum chamber 1. The gas supply device 2 for supplying doping gas containing a desired element such as B2H6 within the vacuum chamber 1 is provided and further the turbo-molecular pump 3 for reducing the pressure within the vacuum chamber 1 is provided, whereby the pressure within the vacuum chamber 1 can be kept at the predetermined value.
Since a high-frequency power is supplied from the power supply 10 to the sample electrode 6 via the capacitor 45 and a high-frequency current transformer 47, plasma is formed within the vacuum chamber 1 and boron ions within the plasma are introduced in the surface of the sample 9. The density of boron thus doped can be controlled by measuring a high-frequency current at the time of discharge by using an ampere meter 48 via the high-frequency current transformer 47. A counter electrode 30 is provided so as to oppose to the sample electrode and the counter electrode 30 is grounded (see a patent document 2, for example).
As another method of controlling a doping quantity a method using Faraday cups is proposed. FIG. 10 shows the schematic configuration of an example of an apparatus for realizing such a method. In FIG. 10, a plasma chamber 49 defines a volume 50 surrounded thereby. A platen 51 disposed within the chamber 49 provides a surface for holding a work piece such as a semiconductor wafer 52 thereon. For example, the wafer 52 is placed on the flat surface of the platen 51 and the peripheral edge of the wafer is clamped by the platen. The platen 51 supports the wafer 52 and provides an electric coupling to the wafer 52. An anode 53 is disposed within the chamber 49 so as to be spaced from the platen 51. The anode 53 is movable in a direction shown by an arrow 54 vertically with respect to the platen 51. Typically, the anode 53 is coupled to the electrically conductive wall of the chamber 49, and both the anode and the wall are grounded. Each of the wafer 52 and the anode 53 is coupled to a high-voltage generator 55 and so the wafer 52 acts as a cathode. Typically, the pulse generator 55 supplies pulses having a voltage range almost from 100 to 500 volt, an interval in a rang almost from 1 to 50 micro second, and a repetition rate in a range almost from 100 to 2 kHz. The volume 50 surrounded by the chamber 49 is coupled to a vacuum pump 57 via a controllable valve 56. A gas source 58 is coupled to the chamber 49 via a mass flow-rate controller 59. A pressure sensor 60 disposed within the chamber 49 supplies a signal representing a pressure within the chamber to a controller 61. The controller 61 compares the detected pressure within the chamber with an inputted desired pressure to apply a control signal to the valve 56. The control signal controls the valve 56 so as to minimize a difference between the pressure within the chamber and the desired pressure. The vacuum pump 57, valve 56, pressure sensor 60 and controller 61 constitute a closed-loop pressure control system. Typically, although the pressure is controlled in a range almost from 1 mTorr to 500 mTorr, the pressure range is not limited to this range. The gas source 58 supplies ionizable gas containing desired dopant to be doped within the work piece. Examples of the ionizable gas are BF3, N2, Ar, PF5 and B2H6. The mass flow-rate controller 59 adjusts a flow rate when the gas is supplied to the chamber 49. The configuration of FIG. 10 provides a continuous flow of process gas with a constant gas flow rate and a constant pressure. The pressure and the gas flow rate are adjusted preferably so as to provide repeatable results.
Upon operation, the wafer 52 is placed on the platen 51. Thereafter, the pressure control system, the mass flow-rate controller 59 and the gas source 58 generate the gas flow of the desired pressure and flow rate. As an example, the chamber 49 operates with BF3 gas at the pressure of 10 mTorr. The pulse generator 55 applies high-voltage pulses continuously to the wafer 52 to form plasma 62 between the wafer 52 and the anode 53. As known as a well-known technique, the plasma 62 contains positive ions of ionizable gas supplied from the gas source 58. The plasma 62 further contains a plasma sheath 63 near the platen 51. During the high-voltage pulse, the magnetic field existing between the anode 53 and the platen 51 accelerates the positive ions so as to move toward the platen 51 from the plasma 62 across the plasma sheath 63. The accelerated ions are implanted within the wafer 52 so as to form a region of impurity material. The pulse voltage is selected so as to implant the positive ions at a desired depth within the wafer 52. The number and interval of pulses are selected so as to provide the impurity material of a desired dose amount to the wafer 52. A current per single pulse is a function of the pulse voltage, the gas pressure, species and the movable position of the electrode. For example, the distance between the cathode and the anode can be adjusted with respect to different voltages.
At least one Faraday cup is disposed adjacent to the platen 51 in order to measure the dose amount of the ions implanted into the wafer 52. In FIG. 10, Faraday cups 64 and 65 are disposed at the peripheral portions of the outer periphery of the wafer 52 with the same interval therebetween. Each of the Faraday cups is formed by a conductive enclosure having an inlet 66 facing on the plasma 62. Preferably, each of the Faraday cups is disposed near the wafer 52 as close as possible actually so as to shield samples of the positive ions accelerated toward the platen 51 from the plasma 62. The Faraday cups are electrically coupled to a dose amount processor or another dose amount monitoring circuit. As known as a well-known technique, the positive ions entered into the Faraday cups via the inlet 66 generates a current within the electric circuit coupled to the Faraday cups. The current represents the number of the positive ions received per a unit time, that is, an ion current. The ion current received by the Faraday cups 64 and 65 is supposed to have a constant relation to the number of ions implanted into the wafer 52. Depending on the uniformity of the plasma 62 and the uniformity of the acceleration of the ions toward the platen 51, the ion current per a unit area received by each of the Faraday cups is substantially equal to the ion current per a unit area implanted into the wafer 52 or a constant part thereof. Since an output current from each of the Faraday cups represents the ion current implanted into the wafer 52, each of the Faraday cups 64 and 65 provides the measurement result of the ion dose amount implanted into the wafer 52. The Faraday cups 64 and 65 may be disposed within a guard ring 67 in the vicinity of the wafer 52 and the platen 51. The electric current representing the ion current is supplied to the dose amount processor 68 from the Faraday cups (see a patent document 3, for example).
As a still another method of controlling the doping quantity, there is proposed a method for reducing a capacitor current corresponding to a displacement current from the current supplied to the sample electrode. In FIG. 11, the plasma doping chamber defines a sealed volume 50. A platen 51 disposed within a chamber 49 provides a surface for holding a substrate to be processed such as a semiconductors wafer 52. The semiconductors wafer is a mere example of types capable of being a target.
For example, the implantation may be made into metal for a tool, a part for an automobile, a stamp die or plastics. For example, the wafer 52 is fastened against the flat surface of the platen 51 at the periphery thereof. The platen 51 supports the wafer 52 and provides an electric coupling to the wafer 52. An anode 53 is disposed within the chamber 49 so as to be spaced from the platen (cathode) 51. The anode 53 is movable in a direction shown by an arrow 54 vertically with respect to the platen 51. Typically, the anode 53 is coupled to the electrically conductive wall of the chamber 49, and both the anode and the wall are grounded.
Thus, each of the cathode 51 and the wafer 52 is coupled to a high-voltage generator 55. Typically, the pulse generator 55 supplies pulses having a voltage range almost from 100 to 10,000 volt and a time interval in a rang almost from 1 to 100 micro second at a repetition rate in a range almost from 50 to 5 kHz. The sealed volume 50 of the chamber 49 is coupled to a vacuum pump 57 via a controllable valve 56. A gas source 58 is coupled to the chamber 49 via a mass flow-rate controller 59. A pressure sensor 60 disposed within the chamber 49 supplies a signal representing a pressure within the chamber to a controller 61. The controller 61 compares the detected pressure within the chamber with an inputted desired pressure to apply a control signal to the valve 56. The control signal controls the valve 56 so as to minimize a difference between the pressure within the chamber and the desired pressure. The vacuum pump 57, valve 56, pressure sensor 60 and controller 61 constitute a closed-loop pressure control apparatus.
Typically, although the pressure is controlled in a range almost from 1 mTorr to 500 mTorr, the pressure range is not limited to this range. The gas source 58 supplies ionizable gas containing desired dopant to be doped within the substrate to be processed. Examples of the ionizable gas are BF3, N2, Ar, PF5 and B2H6. The mass flow-rate controller 59 adjusts a speed of the gas to be supplied to the chamber 49. The configuration of FIG. 11 provides a continuous flow of process gas with a constant gas flow rate and a constant pressure. The pressure and the gas flow rate are adjusted preferably so as to provide repeatable results.
During the operation, the wafer 52 is placed on the platen 51. Thereafter, the pressure control system, the mass flow-rate controller 59 and the gas source 58 are set so as to generate the gas flow of the desired pressure and flow rate. As an example, the chamber 49 operates with BF3 gas at the pressure of 10 mTorr. The pulse generator 55 applies high-voltage pulses continuously to the wafer 52 to form plasma 62 between the wafer 52 and the anode 53. As known as a well-known technique, the plasma 62 contains positive ions of ionizable gas supplied from the gas source 58. Further, the plasma 62 contains a plasma sheath 63 near the platen 51. During the high-voltage pulse, the magnetic field existing between the anode 53 and the platen 51 accelerates the positive ions so as to move toward the platen 51 from the plasma 62 across the plasma sheath 63. On the contrary, secondary electrons that are generated by the ion collision on the platen 51 and the wafer 52 are accelerated into the plasma across the plasma sheath 63. The accelerated ions are implanted within the wafer 52 so as to form a region of impurity material. The pulse voltage is selected so as to implant the positive ions at a desired depth within the wafer 52. The number and interval of pulses are selected so as to provide the impurity material (positive ions) of a desired dose amount within the wafer 52. A current per single pulse is a function of the pulse voltage, the gas pressure, gas species and the position of the electrode. For example, the distance between the cathode and the anode can be adjusted with respect to different voltages.
In the plasma doping apparatus, the pulse current is measured in order to provide a pulse current signal representing an ion current to be distributed to the wafer. The pulse current flowing into the plasma doping apparatus is a sum of the ion current, a secondary electron current and a displacement current. A compensation signal representing the displacement current component of the pulse current is generated. The compensation signal is subtracted from the pulse current signal so as to provide an ion current signal representing the ion current to be distributed to the wafer. To this end, a secondary capacitor is used which preferably has the same or almost same capacitance as that of a primary capacitor constituted by the wall of the chamber and the electrodes of the plasma doping apparatus including the anode and the cathode. When the same or similar voltage pulse is applied to the secondary capacitor, a secondary displacement current is generated which is the same as or quite similar to a primary displacement current generated in the primary capacitor. As described above, the primary current is the sum of the ion current, the secondary electron current and the primary displacement current. Thus, the ion current and the secondary electron current can be measured accurately by subtracting the secondary displacement current (substantially same as the primary displacement current) from the primary current. The secondary capacitor is a variable capacitor which capacitance varies so as to coincide with the capacitance of the plasma doping apparatus.
As shown in FIG. 11, it is considered that a complete anode construction configured by the anode 53 and the wall of the chamber 49 and the platen (cathode) 51 of the plasma doping apparatus constitutes the primary capacitance C1. Preferably, the secondary capacitor C2 is selected so as to have the same or similar capacitance as that of the primary capacitance C1. The primary capacitance C1 and the secondary capacitor C2 are coupled in parallel to each other, effectively.
As clear from this figure, the high voltage pulses from the pulse source 55 are applied to both the capacitors C1 and C2. A first current measurement device 69 is provided so as to measure a current flowing into the primary capacitance C1 and to output the measured value to a dose processor 68. A second current measurement device 70 is provided so as to measure a current flowing into the secondary capacitance C2. The output of the current measurement device 70 is also applied to the dose processor 68. The current measurement devices 69 and 70 are the same type of devices or different types of devices. Pearson coils are used in order to measure the currents flowing into the respective capacitors during the pulse.
However, at present, many different types of current measurement devices can be utilized. One of these different types of current measurement devices is used to measure the currents. During the operation, when the pulse is applied to the two capacitors, the primary current is generated in the primary capacitance C1, and a secondary displacement current which is substantially same as or quite similar to the primary displacement current component of the primary current is generated in the secondary capacitor C2. The ion current and the secondary electron current being distributed to the target can be measured accurately by subtracting the secondary displacement current from the primary current. The accurate measurement of these currents provides better process control and repetitive property (see a patent document 4, for example).
In the case of forming a MOS transistor, for example, prior to the introduction of impurities, a thin oxide film is formed on the surface of a sample within a predetermined oxidizing atmosphere, and then a gate electrode is formed by utilizing such a CVD apparatus. Then, the MOS transistor is obtained by introducing the impurities by using the gate electrode as a mask. However, since a transistor can no be constituted by merely introducing the impurities by the plasma doping processing, it is required to perform an activating processing. The activating processing is to heat and recrystallize a layer in which impurities is introduced by using methods such as a laser annealing or a flash lamp annealing. In this case, a shallow activation layer can be obtained by effectively heating a quite thin layer in which impurities is introduced. In order to effectively heat the quite thin layer in which impurities is introduced, prior to the introduction of the impurities, a processing is performed in advance as to the quite thin layer in which the impurities is to be introduced in order to increase the absorption rate of light irradiated from a light source such as a laser or a lamp. This processing is called as a pre-amorphization. The pre-amorphization is performed in a manner that in a plasma processing apparatus having the similar configuration as that of the aforesaid plasma processing apparatus, plasma such as He gas is generated, and then ions such as the generated He ions are accelerated by a bias voltage toward a substrate and collided against the substrate to break the crystal structure of the surface of the substrate to amorphize. This processing has been proposed by the inventors of this application etc. (See a Non-patent document 1, for example).
Patent document 1: U.S. Pat. No. 4,912,065
Patent document 2: Japanese Patent No. 2,718,926
Patent document 3: JP2003-T-515945
Patent document 4: JP2004-T-513439
Non-patent document 1: Y Sasaki et al., “B2H6 Plasma Doping with In-situ He Pre-amorphyzation”, 2004, Symposia on VLSI Technology and Circuits